I'm not sure how a weapon that powers up in direct sunlight is helpful when fighting vampires. It provides extra punch when you least need it.
Additional ideas:
Specific to vampire-hunting, how about something that locates vampires? Or you could have a weapon that reveals to the wielder where a vampire's crypt is located when it does a critical.
Protections against vampiric abilities and possibly magic in general.
The X of Holy Light: Light spells cast by the wielder do damage to undead (and possibly demons) as a ball attack using the Shock Bolt table (or something more dangerous if you want to power it up) when they are cast (a constant effect on undead would be another power up, but probably too much unless you anticipate Darkness spells in plentiful response, in which case, it mostly just keeps enemy spell-casters tied up).
The X of Sacred Flame: Fire attacks by the wielder are Slaying against undead (who aren't immune to fire, of course).
Potion of Sanctified Veins: For [duration], the imbiber's blood is poisonous to vampires or other unholy things. Any bite attack that scores a critical on imbiber causes such a creature to suffer an A (Holy ?) critical, unless it is a blood-draining attack, in which case the critical is raised to a C critical. These criticals ignore normal critical reduction effects.
The Staker: A heavy crossbow modified to fire a full-fledged stake, with reduced range, but possibly enchanted with a number of useful abilities.